Optimal way to fill terrain between roads
You could:
Create your road with Array and Curve modifiers:

Extrude the outer and inner borders of the road and AltS to scale. Fill the inner face, create a plane all around:

Make the road invisible with H

Create a second object, a plane that you subdivide and that you put over your road, delete the faces only:

Go in the top orthographic view, select the grid, shift select the road, use the Knife Project tool to print the grid on the road object. Unhide the road vertices, here is what you should get:

Now you can shrinkrap the object to another surface. If you don't want the road to shrinkwrap, create a vertex group that you exclude from the Shrinkwrap modifier. You can also sculpt the object, if you want to keep the road flat, hide it before switching to Sculpt mode.
Edit:
How to make the square plane around the road: Extrude the vertex, F to fill between two vertices, F to fill the whole face: